Fuel cell system with degradation protected anode

ABSTRACT

The present invention provides a method of operating a fuel cell including an anode, a cathode, a first passage, and a second passage, wherein the anode is disposed in the first passage and the cathode is disposed in the second passage, comprising: producing a non-explosive gaseous feed consisting of (i) at least one oxidizable component having a greater tendency to undergo oxidation relative to the anode, and (ii) a remainder, wherein the remainder is the predominant component in the gaseous feed and consists essentially of water vapor, and introducing the non-explosive gaseous feed to the first passage to form a first gaseous feed stream flowing through the first passage when the anode realizes a temperature effective to facilitate deteriorative oxidation of the anode in the presence of an oxidizing agent. The non-explosive gaseous feed is provided to mitigate or prevent anode oxidation and to mitigate or prevent the formation of potentially explosive gaseous mixtures. Additionally, the non-explosive gaseous feed can provide a source of steam for reforming.

FIELD OF INVENTION

[0001] This invention relates to fuel cell systems, and particularly tosolid oxide fuel cells with systems configured to mitigate deleteriousoxidation of fuel cell components.

BACKGROUND OF THE INVENTION

[0002] Solid oxide fuel cells typically operate at high temperatureconditions. Because of these temperature conditions, solid oxide fuelcells typically require a supply of purge gas during various stages ofoperation. During start-up and shutdown of a solid oxide fuel cell, itis preferable not to flow gaseous fuel through a fuel cell, as it ispotentially explosive at these lower temperatures. However, without themaintenance of a reducing environment in the region of the anode, theanode may be susceptible to oxidation, thereby compromisingelectrochemical performance and/or service life of the fuel cell. Assuch, during start-up and shut down conditions, it is not only importantto avoid exposure of solid oxide fuel cell components to an atmospherewhich is oxidizing, but it is also important to prevent their exposureto a potentially explosive atmosphere.

[0003] To mitigate these conditions, purge gas systems have beendeveloped to supply solid oxide fuel cells with purge gas duringstart-up and shutdown conditions. An example of such a system isdescribed in U.S. Pat. No. 5,928,805 issued to Singh et al. In Singh etal., the purge gas is generated by combusting a hydrocarbon fuel in thepresence of air to generate a non-explosive mixture of combustionproducts, to which stored hydrogen is selectively added to maintain thedesired hydrogen concentration in the final gas stream entering thesolid oxide fuel cell. To generate the desired cover gas composition inthis case, the flow of reactants entering the burner must be carefullycontrolled in order to form a combustion product whose hydrogen contentis able to be attenuated to a desired level by selective addition ofhydrogen from a separate supply source.

SUMMARY OF THE INVENTION

[0004] The present invention provides a method of operating a solidoxide fuel cell with a first gaseous fluid comprising water vapor and atleast one oxidizable component characterized by a greater tendency toundergo oxidation relative to the anode.

[0005] In one aspect, the present invention provides a method ofoperating a fuel cell including an anode, a cathode, a first passage,and a second passage, wherein the anode is disposed in the first passageand the cathode is disposed in the second passage, comprising: producinga non-explosive gaseous feed consisting of at least one oxidizablecomponent having a greater tendency to undergo oxidation relative to theanode, and a remainder, wherein the remainder is the predominantcomponent in the gaseous feed and consists essentially of water vapor,and introducing the non-explosive gaseous feed to the first passage toform a first gaseous stream flowing through the first passage when theanode realizes a temperature effective to facilitate deteriorativeoxidation of the anode in the presence of an oxidizing agent.

[0006] The concentration of the water vapor in the gaseous feed may begreater than 50% by volume based on the total volume of the gaseousfeed. The concentration of the at least one oxidizable component is lessthan the minimum concentration necessary to render the gaseous feedpotentially explosive at the effective temperature. In this respect, theconcentration of the at least one oxidizable component may be less thanthe lower flammability limit of the at least one oxidizable component.The concentration of the at least one oxidizable component may beeffective to mitigate, or substantially prevent, deteriorative oxidationof the anode. The at least one oxidizable component may be selected fromthe group consisting of hydrogen, alcohols, aldehydes, ketones, ammonia,hydrazine, and hydrocarbons. The method may further comprise evaporatingan aqueous mixture consisting essentially of water and at least oneoxidizable component to produce the gaseous feed. The anode may comprisenickel. Where the anode comprises nickel, the temperature effective tofacilitate deteriorative oxidation of the anode is greater than or equalto 400° C. The method may further comprise flowing a second gaseousstream through the second passage, the second gaseous stream includingoxygen, while contemporaneously flowing the first gaseous stream throughthe first passage. The at least one oxidizable component may bemethanol, and the concentration of methanol in the aqueous solution maybe less than about 2.4% by weight based on the total weight of theaqueous solution. It is understood that, in the typical case thetemperature effective to facilitate deteriorative oxidation of the anodeis much higher than the boiling temperature of water, therefore thepurge gas mixture can be safely supplied to the fuel cell without thedanger of steam condensation deleteriously affecting its operation.

[0007] In another aspect, the present invention provides a method ofoperating a fuel cell including an anode, a cathode, a first passage,and a second passage, wherein the anode is disposed in the first passageand the cathode is disposed in the second passage, comprising:

[0008] (i) progressively heating the first passage;

[0009] (ii) producing a non-explosive gaseous feed consisting of atleast one oxidizable component having a greater tendency to undergooxidation relative to the anode, and a remainder, wherein the remainderis the predominant component in the gaseous feed and consistsessentially of water vapor; and

[0010] (iii) purging the first passage with the gaseous feed when thetemperature of the anode is above a temperature effective to causedeteriorative oxidation of the anode in the presence of an oxidizingagent.

[0011] In a further aspect, the present invention provides a method foroperating a fuel cell including an anode, comprising nickel, a cathode,a first passage, and a second passage, wherein the anode is disposed inthe first passage and the cathode is disposed in the second passage,comprising: producing a non-explosive gaseous feed comprising watervapor and at least one oxidizable component having a greater tendency toundergo oxidation relative to the anode by either of (a) evaporating anaqueous mixture comprising the at least one oxidizable component, or (b)evaporating a source of water to produce the water vapor, and combiningthe water vapor with the at least one oxidizable component. Theevaporation may be a flash evaporation. When the temperature within thefirst passage is sufficiently high such that the gaseous fuel is notpotentially explosive when disclosed in the first passage, the purgingof the first passage by the gaseous feed may be terminated and a gaseousfuel can then be flowed through the first passage.

[0012] In a further aspect, the present invention provides a fuel cellsystem comprising: a fuel cell including an anode, a cathode, a firstpassage, and a second passage, wherein the anode is disposed in thefirst passage and the cathode is disposed in the second passage; meansfor evaporating an aqueous mixture including at least one oxidizablecomponent to form a gaseous feed, and means for delivering the gaseousfeed to the first passage to form a first gaseous stream flowing throughthe first passage and effective in mitigating corrosion of the anode.

[0013] In yet another aspect, the present invention provides a fuel cellsystem comprising a fuel cell including an anode, a cathode, a firstpassage, and a second passage, wherein the anode is disposed in thefirst passage and the cathode is disposed in the second passage, anevaporator for a fuel cell system comprising: a fuel cell including ananode, a cathode, a first passage, and a second passage, wherein theanode is disposed in the first passage and the cathode is disposed inthe second passage: an evaporator, fluidly communicating with the firstpassage, and configured to evaporate an aqueous mixture including atleast one oxidizable component to form a gaseous feed, and a controller,communicating with the fuel cell for receiving an anode corrosionindication, for effecting delivery of the gaseous feed to the firstpassage to form a first gaseous stream flowing through the first passagein response to the anode corrosion indication within the fuel cell.

[0014] In one aspect, the controller is coupled to a temperature sensorfor measuring a temperature within the fuel cell, wherein the controlleris configured to effect the delivery of the gaseous feed at apredetermined temperature. The controller may also be coupled to amotive means configured to effect the delivery of the gaseous feed,wherein the controller is configured to actuate the motive means toeffect the delivery of the gaseous feed at a predetermined temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] This invention will be better understood by reference to thefollowing detailed description of the invention in conjunction with thefollowing drawings, in which:

[0016]FIG. 1 is a schematic illustration of a fuel cell; and

[0017]FIG. 2 is a schematic illustration of an embodiment of the fuelcell system of the present invention.

DETAILED DESCRIPTION

[0018] Referring to FIG. 1, a solid oxide fuel cell 110 includes a firstpassage 112 and a second passage 114. The first passage 112 is separatedfrom the second passage 114 by a substantially ionically conductingseparator 116 to permit selective permeation of ionic speciestherethrough. The term “substantially ionically conducting” recognizesthat the separator 116 conducts electricity to a small degree but not tothe extent where it significantly impacts on performance of the fuelcell 110 (due to the fact that the conductance of electrons through theseparator 116 short-circuits the electrodes 118, 120, described below).An anode 118 is disposed in the first passage 112, and a cathode 120 isdisposed in the second passage 114. Each of the anode 118 and thecathode 120 is disposed in intimate contact with the separator 116 forfacilitating migration of ionic species. Each of the anode 118 and thecathode 120 is also electrically connected to an external load 122,thereby facilitating the conductance of electrons between the electrodesand to an external load 122.

[0019] Gaseous fuel is flowed through the first passage 112. An oxidantcharacterized by a greater tendency to undergo reduction relative to thegaseous fuel is flowed through the second passage 114. The fuel isoxidized at the anode 118, and the oxidant is reduced at the cathode120. An example of a suitable solid oxide fuel cell of the “tubular”variety is disclosed in U.S. Pat. No. 4,395,468. It is understood thatsolid oxide fuel cells of the “planar” or other varieties, or other hightemperature fuel cells, fall within the scope of this invention.

[0020] To mitigate corrosion of components within a fuel cell, whilepreventing the creation of a potentially explosive environment, thepresent invention provides a method of operating a fuel cell. The methodcomprises producing a non-explosive gaseous feed and introducing thegaseous feed to the first passage to form a first gaseous stream flowingthrough the first passage. Gaseous feed can be introduced to the firstpassage to form a first gaseous stream flowing through the first passagefor the purpose of removing and replacing a gaseous volume within thefirst passage. In such case, the introduction of the gaseous feed to thefirst passage for this purpose is referred to as “purging” of the firstpassage. Purging of the first passage can be effected to removeundesirable gaseous substances, such as those substances whose presencein the first passage could increase the risk of formation of anexplosive mixture within the first passage during the operation of thefuel cell. Purging of the first passage can be effected during start-upor shutdown of the fuel cell. During start-up, the first passage ispurged of air. During shutdown, the first passage is purged of fuel.

[0021] The gaseous feed consists of (i) at least one oxidizablecomponent having a greater tendency to undergo oxidation relative to theanode, and (ii) a remainder. The remainder is the predominant componentin the gaseous feed. Being the predominant component means that themajority of the volume of the gaseous feed consists of the remainder.The concentration of the at least one oxidizable component of thegaseous feed is outside the concentration necessary to render thegaseous feed, or the first gaseous stream, potentially explosive at theeffective temperature. In this respect, the gaseous feed isnon-explosive.

[0022] Upon the anode realizing a temperature effective to facilitatedeteriorative oxidation of the anode in the presence of an oxidizingagent (the “effective temperature”), a reducing atmosphere, relative tothe anode, is required in the first passage. From a thermodynamicperspective, at temperatures lower than this effective temperature, theanode is susceptible to oxidation by an oxidizing agent, such as air,but at an acceptably low reaction rate. The rate of oxidation, however,increases with temperature. At the effective temperature, the rate ofoxidation is unacceptably high and leads to structural deterioration ofthe anode to the point where electrochemical performance and/or servicelife of the fuel cell is compromised, and is what is referred to hereinas “deteriorative oxidation”.

[0023] In one embodiment, the anode comprises nickel, and the effectivetemperature at which deteriorative oxidation of the anode and/or itscomponents becomes of concern is tied to the susceptibility of nickel todeteriorative oxidation. In the presence of oxygen or other oxidizingagents, and at temperatures above 400° C., nickel is susceptible tounacceptably high rates of oxidation. From a thermodynamic perspective,nickel is susceptible to oxidation by air at any temperature, but thereaction rate is very low at low temperatures. The oxidation rateincreases with temperature. Usually an oxide film forms on the surfacethat slows down further oxidation or brings it nearly to zero, and bulknickel does not get easily oxidized up to a relatively high temperature.In a solid oxide fuel cell anode, nickel is usually present in a form ofvery small particles dispersed in a porous ceramic matrix, such asyttria stabilized zirconia. As the surface area of nickel is very high,and access thereto by oxygen is relatively good, the sensitivity ofnickel to oxidation is relatively high. Oxidation converts nickel toless dense nickel oxides (primarily NiO), thereby creating space demandsand creating stresses in the structure up to the point ofdisintegration. This phenomenon becomes more pronounced at highertemperatures and results in deteriorative oxidation. However, there isno clear temperature above which nickel must be protected from, as thismay depend on the particular form of nickel, the overall composition ofthe anode and the duration of exposure. Providing a reducingenvironment, relative to nickel, in the first passage at a temperatureof 400° C. is prudent for most circumstances, but depending on theparticular conditions, a higher starting temperature may be selected. Alower starting temperature may be necessary for a fuel cell that usespure oxygen or operates at a relatively high pressure.

[0024] To create the desired reducing atmosphere at the effectivetemperature, the gaseous feed is introduced to the first passage to forma first gaseous stream flowing through the first passage. The gaseousfeed consists of at least one oxidizable component and a remainder,wherein the remainder consists essentially of water vapor. The at leastone oxidizable component is defined as a compound having a greatertendency to undergo oxidation relative to the anode. In this respect,the oxidizable component in the gaseous feed is more likely to beoxidized in the presence of an oxidizing agent, relative to the anode orany component of the anode, such as one or more components of thesurface of the anode. Conversely, a non-oxidizable component in thegaseous feed is less likely, or as likely, to be oxidized in thepresence of an oxidizing agent, relative to the anode or any componentof the anode, such as one or more components of the surface of theanode. The term “oxidizing agent” refers to a compound that has agreater tendency to undergo reduction, relative to the anode or anycomponent of the anode, such as one or more components of the surface ofthe anode.

[0025] For example, where the anode comprises nickel, suitableoxidizable components include hydrogen, alcohols, aldehydes, ketones,ammonia, hydrazine, esters, organic acids, suitable hydrocarbons, and,in general, any organic compound that can be substantially converted tooxidizable gaseous products during evaporation of their aqueous mixturesand/or subsequent reactions as the suitable oxidizable components andits reaction product(s) are delivered to the fuel cell 110 and flowthrough the fuel cell 110. Any of these suitable oxidizable componentsmust be compatible with fuel cell operation (e.g. no deleterious sulphurcontent) and must have a greater thermodynamic tendency to undergooxidation, in the presence of oxygen or any other oxidizing agent, thannickel. Nickel is commonly incorporated in solid oxide fuel cell anodes.

[0026] In one embodiment, the at least one oxidizable component(including one or more of such oxidizable components) is present in aconcentration effective to mitigate deteriorative oxidation of theanode. An inadequate concentration of the at least one oxidizablecomponent may be insufficient to mitigate deteriorative oxidation of theanode where an oxidant is present in the environment immediate to theanode. Deteriorative oxidation of the anode includes reference to theoxidation of one or more components of the anode surface that impactsthe performance or service life of the fuel cell. In one embodiment, oneanode surface component is nickel. Mitigation of deteriorative oxidationincludes reference to reducing the rate of deteriorative oxidationrelative to the condition where no gaseous stream is flowed through thefirst passage, and the anode is exposed to an atmosphere including anoxidizing agent. The rate of deteriorative oxidation is reduced byintroducing the gaseous feed to the first passage, and therebyintroducing the at least one oxidizable component to the first passage.Upon introduction to the first passage, the at least one oxidizablecomponent and/or its reaction products are exposed to the anode, andthereby contribute to the creation of a reducing atmosphere relative tothe anode. The reaction products include those resulting from steamreformation and/or thermal decomposition of the at least one oxidizablecomponent while the gaseous feed is being introduced to the firstpassage, and as the at least one oxidizable component, and/or thereaction products created during the introduction, flows through thefirst passage as part of the first gaseous stream.

[0027] In another embodiment, the at least one oxidizable component ispresent in a concentration effective to substantially preventdeteriorative oxidation. In practical terms, the deteriorative oxidationof the anode is substantially prevented if the corrosion negligiblyimpacts on the electrochemical performance or the service life of thefuel cell.

[0028] The at least one oxidizable component enjoys a lower oxidationpotential than does the anode, or any of the components of the anode.Oxidation potential is defined as the tendency of a substance to acceptelectrons. In general, it can be quantified as an electrical potentialof an inert metallic electrode in equilibrium with both the reduced andoxidized forms of the substance, measured with reference to a standardelectrode. It is a measure of the tendency of a particular compound tobe reduced, or the tendency to oxidize other substances. A compoundcharacterized by a lower oxidation potential is more likely to beoxidized versus compounds characterized by higher oxidation potentials.

[0029] As mentioned above, the remainder of the gaseous feed consistsessentially of water vapor. In this respect, compound(s) other thanwater vapor and the at least one oxidizable component may be present inthe gaseous feed in small amounts which are not sufficiently significantto derogate from the cost savings associated with using water vapor as asignificant inert component of the gaseous feed, as well as derogatefrom the reducing atmosphere intended to be created within the firstpassage by introducing the gaseous feed to the first passage. Suchcompounds include reaction products which may be formed while thegaseous feed is being introduced to the first passage, including anynon-oxidizable components formed during reformation of the at least oneoxidizable component, as the gaseous feed is being flowed to the firstpassage during the introduction of the gaseous feed to the firstpassage. In one embodiment, the concentration of water vapor in thegaseous feed is greater than 50% by volume based on the total volume ofthe gaseous feed.

[0030] For safety considerations, the gaseous feed must be non-explosiveat the temperature effective to facilitate deteriorative oxidation ofthe anode. In this respect, the concentration of the at least oneoxidizable component of the gaseous feed is lower than the concentrationnecessary to render the gaseous feed, or the first gaseous stream,potentially explosive at these temperatures. For example, where thegaseous feed consists of water vapor and hydrogen, and where thetemperature of the gaseous feed or the first gaseous stream is below theautoignition temperature for hydrogen (i.e. 590° C.), it would beprudent to maintain the hydrogen concentration in the gaseous feed belowa concentration which, when mixed with an oxidizing agent source, suchas air, forms a potentially explosive mixture. Such an upperconcentration limit is known as the lower flammability limit. Theflammability limits for hydrogen in air is 4.0% to 75% by volume, basedon the total volume of the hydrogen-air mixture at ambient temperatureand pressure. As such, a “safe” hydrogen concentration in the gaseousfeed is 4.0% by volume, based on the total volume of the gaseous feed,acknowledging that a generous safety factor is built into this limitingsafe concentration, as the hydrogen would only be further diluted uponmixing with air. In this respect, in some instances, higher hydrogenconcentrations could be employed, such as 5% by volume, withoutcompromising safety. Flammability limits are pressure and temperaturedependent and can expand with an increase in pressure and/or temperatureof the gaseous feed. As such, the “safe” hydrogen concentration in thegaseous feed can be lower than 4% by volume at pressures and/ortemperatures higher than ambient.

[0031] The gaseous feed is introduced to the first passage, to produce afirst gaseous stream flowing through the first passage, until atemperature is reached where it is safe to flow a gaseous fuel throughthe first passage. Below this threshold temperature, the gaseous fuel ispotentially explosive. A gas explosion is generally defined as a processwhere combustion of a premixed gas, i.e. fuel-air or fuel-oxidizer,causes a rapid increase in pressure. Above this threshold temperature,the gaseous fuel burns without exploding. Where the gaseous fuelincludes hydrogen at a concentration of over 4% by volume, based on thetotal volume of the fuel, and the anode comprises nickel, it is prudentto flow the first gaseous stream through the first passage when thetemperature of the anode is above 400° C. in order to protect the anodefrom corrosion. It is unsafe to flow the gaseous fuel through the firstpassage at temperatures below the autoignition temperature of thegaseous fuel, because the gaseous fuel includes hydrogen at aconcentration that could create a potentially explosive mixture. Wherethe temperature in the first passage is above the autoignitiontemperature of the gaseous fuel, it may be safe to flow the gaseous fuelthrough the first passage, without the risk of explosion. However, whereanodes require protection from oxidation at temperatures below theautoignition temperature, it is necessary to flow a first gaseous streamderiving from the gaseous feed, instead of the gaseous fuel, through thefirst passage, to avoid potentially explosive conditions.

[0032] A gaseous fuel is defined as a substance or a mixture ofsubstances in the form of gaseous or gaseous vapors that can undergooxidation by reacting directly or indirectly (e.g., in anelectrochemical cell) with oxygen, with concomitant production ofthermal or electrical energy. With reference to the operation of solidoxide fuel cells, the defining feature of the gaseous fuel is the formunder which the substance or the mixture of substances undergooxidation, and particularly the form in which the substance or mixtureof substances react at the fuel cell anode to produce electrical energy,and not the form of the substance or mixture of substances that are fedto the fuel cell system and result in the particular gaseous fuelcomposition being present in the fuel cell stack and undergoingoxidation at the fuel cell anodes. Thus, for example, with the primaryfuel being liquid hydrocarbon or liquid mixture of hydrocarbons such asgasoline, naphtha or diesel fuel, the ultimate substance or mixture ofsubstances being oxidized in the fuel cell stack and reacting at theanode are gaseous phases formed by evaporation or chemical conversion ofthe primary liquid fuel.

[0033] In one embodiment, a gaseous feed is introduced to the firstpassage, wherein the gaseous feed consists of methanol and a remainder,wherein the remainder consists essentially of water vapor. Theconcentration of methanol in the gaseous feed is insufficient to renderthe first gaseous stream in the first passage a potentially explosivemixture. In this respect, it is prudent to maintain the methanolconcentration below that concentration which, when completely reformedinto hydrogen, would produce a gaseous mixture whose hydrogenconcentration is an acceptable hydrogen concentration, as explainedabove. Methanol is reformed to hydrogen, to some extent, as the gaseousfeed is being produced by flash evaporation and being delivered to thefirst passage, and as the components of the gaseous feed and itsreaction products pass through the first passage. In one embodiment, areformer catalyst can be provided within the first passage, or anexternal feed conduit coupled to the first passage, to enhance methanolreformation. In one embodiment, and where a conservative system designis desired, it is preferred to assume that all of the methanol isreformed to hydrogen when attempting to determine a threshold methanolconcentration.

[0034] For example, a gaseous feed comprising 2.4% methanol by weight,based on the total weight of the gaseous feed, when completely reformed,is transformed into a gaseous mixture comprising 4.0% hydrogen byvolume, based on the total volume of the gaseous mixture (assuming thatmethanol is the only component of the initial gaseous feed that can be asource of hydrogen, upon reforming). As explained above, in oneembodiment, when it is desired to operate with a safety margin, anacceptable threshold hydrogen concentration is 4.0% by volume at ambientpressure and temperature, based on the total volume of the gaseousmixture. Accordingly, an acceptable threshold methanol concentration inthe gaseous feed, given this safety margin, is 2.4% by weight, based onthe total weight of the gaseous feed, assuming all of the methanol isreformed into hydrogen. Of course, higher methanol concentrations maystill be acceptable, depending on acceptable risk tolerances.

[0035] To form the first gaseous stream flowing through the firstpassage, the gaseous feed is produced and then introduced to the firstpassage. In this respect, in one embodiment, the gaseous feed isproduced by an evaporator by way of flash evaporation of an aqueoussolution comprising at least one oxidizable component and water. Wherethe at least one oxidizable component is a liquid hydrocarbon, thegaseous feed is generated from an evaporator by way of flash evaporationof an emulsion, preferably a stable emulsion, comprising an hydrocarbonand water. In one embodiment, the emulsion comprises diesel fuel inwater. Aqueous solutions and aqueous emulsions are collectively referredto herein as “aqueous mixtures”.

[0036] Flash evaporation facilitates simpler process control, as thecomposition of the gaseous feed is substantially the same as that of thestarting aqueous mixture. Further, and in contrast with the slow boilingof a starting aqueous mixture, flash evaporation mitigates theconsequences of initial generation of the gaseous feed enriched in themore volatile oxidizable component, which could increase the risk ofexplosion if the enrichment is significant. Infinitesimal enrichment ofthe at least one oxidizable component in the vapor phase is acceptable,so long as potentially explosive concentrations are not approached.

[0037] The gaseous feed can be produced by means other than flashevaporation of an aqueous mixture comprising an oxidizable component.For example, in the case of a gaseous feed consisting of water vapor andmethanol, each of these gaseous components can be derived by separatelyevaporating these components in their pure liquid state, and thencombining the resultant evaporated streams to achieve the desiredconcentration. As a further example, where the oxidizable component ishydrogen, this component can be derived by desorbing a metal hydridesource of its adsorbed hydrogen constituent, or by directly addinggaseous hydrogen to the stream from a compressed gaseous feed cylinder.

[0038] In one embodiment, during start-up, the first passage isprogressively heated. In this respect, the first passage can be heatedinternally by way of conduction, using an electric heater or combustinga fuel in a region proximate to the first passage. Alternatively, thegaseous feed can be heated externally of the solid oxide fuel cell,prior to its introduction into the first passage, or air can be heatedexternally of the solid oxide fuel cell, prior to its introduction tothe second passage. When the temperature of the anode is above atemperature effective to cause oxidation of the anode, the gaseous feedis introduced to the first passage to form the first gaseous streamflowing through the first passage. Once the temperature within the firstpassage is sufficiently high such that the gaseous fuel is notpotentially explosive when disposed in the first passage, the flow ofthe gaseous feed is terminated, and flow of gaseous fuel into the firstpassage is commenced. It is understood that the flow of the gaseous fuelcan be commenced before the flow of the gaseous feed is terminated, solong as the temperature within the first passage is sufficiently highsuch that the gaseous fuel is not potentially explosive when disposed inthe first passage. In this respect, in one embodiment, when thetemperature of the first passage is sufficiently high, flow of thegaseous fuel through the first passage can be commenced, andprogressively increased while the flow of the gaseous feed isprogressively decreased and, eventually, terminated.

[0039] During start-up or shutdown conditions, a first gaseous streamcomprising an oxidizable component, such as hydrogen, is flowed throughthe first passage, and small quantities of the first gaseous stream mayleak from the first passage and into the second passage, therebyexposing the cathode to the first gaseous stream. Because the oxidizablecomponent has a tendency to undergo oxidation, exposing the cathode tothe first gaseous stream could promote reduction and, therefore,decomposition of the cathode. In order to mitigate cathode decompositionduring start-up or shutdown conditions, it is desirable to flow a secondgaseous stream through the second passage, and across the cathode,wherein the second gaseous stream includes a reducible component with agreater tendency to undergo reduction relative to the cathode.

[0040] In this respect, in another embodiment, and contemporaneouslywith the step of purging the first passage with the gaseous feed, asecond gaseous stream is flowed through the second passage, wherein thesecond gaseous stream includes a reducible component with a greatertendency to undergo reduction relative to the cathode. The secondgaseous stream can be flowed through the second passage of a fuel cellduring start-up or shutdown conditions. Like the anode, the cathode isalso susceptible to corrosion during start-up or shutdown conditions. Inthe case of the cathode, however, decomposition occurs at elevatedtemperatures when the cathode is exposed to a reducing atmosphere (i.e.,the cathode is exposed to a gaseous feed, such as the first gaseousstream, which includes components which have a greater thermodynamictendency to undergo oxidation, relative to the cathode, and therebycause reduction of the cathode).

[0041] For instance, in one embodiment, the cathode comprises at leastsome oxides of lanthanum and other rare earth elements, magnesium,calcium and other alkaline earth elements, chromium, manganese, cobaltand nickel, preferably but not necessarily in the crystallographic formof perovskite (not all elements need to be present in the cathodecomposition, and some may be substituted by elements of similar chemicalproperties). An example of a suitable cathode material is lanthanumstrontium manganate. In complementary relationship to the cathodematerial, a suitable second gaseous stream comprises air or anothergaseous mixture containing oxygen in an amount adequate to preventreduction of the cathode material caused by potential leakage ofoxidizable components from the anode passages as well as any otherreducing conditions encountered during operation of solid oxide fuelcell.

[0042]FIG. 2 is a schematic illustration of a system 10, according tothe present invention. The system 10 includes a fuel cell 12, a storagevessel 14, a pump 16 and a flash evaporator 18. The fuel cell 12includes an anode 20, and a cathode 22. The anode 20 is disposed in afirst passage 24, and the cathode 22 is disposed in the second passage26. The first passage 24 is coupled to a fuel supply 32, and the secondpassage is coupled to an air supply 34.

[0043] The storage vessel 14 contains an aqueous mixture including theat least one oxidizable component. The pump 16 is coupled to the storagevessel 14 and delivers the aqueous mixture to the flash evaporator 18.The flash evaporator 18 effects flash evaporation of the aqueous mixturedelivered by the pump to produce gaseous feed that flows to and isthereby introduced to the first passage 24. Upon introduction to thefirst passage 24, the gaseous feed forms a first gaseous stream flowingthrough the first passage 24 and which is effective in mitigatingdecomposition of the anode 20 upon the anode 20 realizing apredetermined temperature.

[0044] During start-up, the fuel cell 12, including the first passage24, is heated. Heat may be applied by way of conduction, using anelectric heater or combusting a fuel in a region proximate to the fuelcell 12, and including the first passage 24. Alternatively, the gaseousfeed can be heated externally of the fuel cell 12, prior to itsintroduction into the first passage 24, or air can be heated externallyof the fuel cell 12 prior to its introduction to the second passage 26.

[0045] The system 10 also includes a temperature sensor 28 for sensing atemperature representative of the temperature of the anode 20. Thetemperature sensor 28 is connected to a controller 30, such as in thiscase a pump controller, and is configured to transmit a signalrepresentative of the temperature of the anode 20 to the controller 30.

[0046] The controller 30 is configured to actuate operation of the pump16 during start-up, upon receiving a signal from the temperature sensor28 indicative of a first predetermined low temperature level. The firstpredetermined low temperature level is a temperature at which it isdesirable to flow a gaseous feed through the first passage, to mitigatedeteriorative oxidation of the anode. The role of the gaseous feedduring start-up is to mitigate or prevent oxidation of the anode 20 andis also to mitigate or prevent the formation of explosive fuel-air gasmixtures in the fuel cell by purging air from the first passage 24.

[0047] Upon further heating of the fuel cell 12, the first predeterminedhigh temperature level is eventually encountered and defines thetemperature at which the gaseous fuel can be safely supplied to the fuelcell 12 without the risk of explosion. After reaching the firstpredetermined high temperature level, the flow of the gaseous fuel iscommenced through the first passage. After introducing the flow of thegaseous fuel at a rate sufficient to scavenge oxygen which may leak ordiffuse into the anode compartment, the flow of gaseous feed gas is nolonger required for the purpose of protecting the anode 20 fromdeteriorative oxidation. In one embodiment, the operation of the pump 16is stopped. Alternatively, in another embodiment, and depending on therequirements of the particular fuel cell 12, it may be preferable togradually replace the gaseous feed flow with fuel flow by simultaneouslydecreasing the rate of discharge from the pump 16 and progressivelyincreasing the fuel flow, or commencing fuel flow prior to stoppingoperation of the pump 16.

[0048] Where the fuel cell 12 uses integrated fuel reformers (such asthose described in U.S. Pat. Nos. 4,395,468 and 4,374,184), an additionof steam to the fuel may be required during the period immediately afterthe commencement of fuel flow, or during other periods of fuel celloperation for purposes of reforming the fuel. In fuel cells usingintegrated reformers, the exhaust is recirculated to provide a source ofoxygen containing species to the reformers. During start-up or duringperiods when the current generated is low in comparison with the fuelsupply rate, stack exhaust is insufficiently oxidized. In this respect,in another embodiment, the purge gas may be used to supply the fuel cell12 with the required steam. As one variation, the purge gas deliverysystem can be used to supply substantially pure steam by replacing theaqueous mixture in the supply tank 14 with pure water.

[0049] The gaseous feed flow can also be used during fuel cell shutdown.The role of the gaseous feed flow is to protect the anode 20 fromdeteriorate oxidation and also to remove the remnants of the previouslysupplied fuel from the fuel cell 12 to mitigate the risk of explosion.The gaseous feed is supplied to the first passage 24 immediately afterthe supply of fuel is terminated or in anticipation of an imminent fuelsupply shortage. In this respect, during shutdown, and in concert withsensing termination of fuel flow or anticipated low fuel flow, thecontroller 30 is configured to actuate operation of the pump 16 uponreceiving a signal from the temperature sensor 28 indicative of a secondpredetermined high temperature level, at which it is desirable to ceaseflow of the fuel (as the risk of explosion is unacceptable) and commenceflow of the gaseous feed. During shutdown, the controller 30 is alsoconfigured to stop operation of the pump 16 upon receiving a signal fromthe temperature sensor 28 indicative of a second predetermined lowtemperature level, at which flow of the gaseous feed is unnecessary asdeteriorative oxidation of the anode 20 is not of concern.

[0050] The present invention will be further described with reference tothe following non-limitative examples.

EXAMPLE NO. 1

[0051] A planar solid oxide fuel cell supplied by InDEC b.v. (P.O. Box1, 1755 ZG PETTEN, The Netherlands) of the “anode supported” type (thethickest and structural layer being the anode) was held at 800° C. in anatmosphere of air at the cathode side and gaseous feed mixture of 3%hydrogen in nitrogen at the anode side. The anode side N₂—H₂ gaseousfeed mixture was then replaced with a mixture of steam and methanolvapors formed by flash evaporation of 3-weight % solution of methanol inwater. The temperature of the cell was maintained at 800° C. for severalhours, and then allowed to gradually cool to room temperature with theanode remaining in the steam-methanol vapors atmosphere, while at thetemperature above 400° C. The following observations were made:

[0052] (1) The cell voltage in the presence of steam-methanol mix thatwas measured for the cell at the temperature above 400° C. remainedsubstantially higher than the cell voltage corresponding to theconditions of anode oxidation. This indicated good protective propertiesof the gaseous feed mixture.

[0053] (2) After cooling down to room temperature, the cell did not showany sign of anode oxidation or degradation that could be associated withan oxidation of anode. Results of similar tests carried out on othercells without the presence of protective atmosphere (such as nitrogen-3%hydrogen) showed a significant level of anode damage most often leadingto mechanical disintegration of the cell.

EXAMPLE NO. 2

[0054] A tubular solid oxide fuel cell as manufactured by SiemensWestinghouse Power Corporation (1310 Beulah Road, Pittsburgh Pa.,15235-5098) was operated at around 1000° C. in an atmosphere of air atthe cathode side and gaseous fuel mixture containing 11% of water vaporsin hydrogen at the anode side. The electrochemical discharge of the cellwas then terminated and the cell brought to open-circuit conditions. Theanode side H₂—H₂O gaseous feed mixture was replaced with a mixture ofsteam and ethanol vapors produced by flash evaporation of 0.6-mol/litersolution of ethanol in water. The cell was allowed to cool under thepurge of the steam-ethanol vapor mixture. The subsequent analysis didnot indicate any sign of oxidation damage to the anode or to the cell.

[0055] Although the disclosure describes and illustrates preferredembodiments of the invention, it is to be understood that the inventionis not limited to these particular embodiments. Many variations andmodifications may occur to those skilled in the art within the scope ofthe invention. For definition of the invention, reference is to be madeto the appended claims.

1. A method of operating a fuel cell including an anode, a cathode, afirst passage, and a second passage, wherein the anode is disposed inthe first passage and the cathode is disposed in the second passage,comprising: (i) producing a non-explosive gaseous feed consisting of (i)at least one oxidizable component having a greater tendency to undergooxidation relative to the anode, and (ii) a remainder, wherein theremainder is the predominant component in the gaseous feed and consistsessentially of water vapor; and (ii) introducing the non-explosivegaseous feed to the first passage to form a first gaseous stream flowingthrough the first passage when the anode realizes a temperatureeffective to facilitate deteriorative oxidation of the anode in thepresence of an oxidizing agent.
 2. The method as claimed in claim 1,wherein the concentration of the water vapor in the gaseous feed isgreater than 50% by volume based on the total volume of the gaseousfeed.
 3. The method as claimed in claim 2, wherein the concentration ofthe at least one oxidizable component is less than the minimumconcentration necessary to render the gaseous feed potentially explosiveat the effective temperature.
 4. The method as claimed in claim 2,wherein the concentration of the at least one oxidizable component isless than the lower flammability limit of the at least one oxidizablecomponent.
 5. The method as claimed in claim 2, wherein theconcentration of the at least one oxidizable component is effective tomitigate deteriorative oxidation of the anode.
 6. The method as claimedin claim 5, wherein the concentration of the at least one oxidizablecomponent is effective to substantially prevent deteriorative oxidationof the anode.
 7. The method as claimed in claim 6, wherein the at leastone oxidizable component is selected from the group consisting ofhydrogen, alcohols, aldehydes, ketones, esters, organic acids, ammonia,hydrazine, and hydrocarbons.
 8. The method as claimed in claim 7,further comprising evaporating an aqueous mixture consisting essentiallyof water and the at least one oxidizable component to produce thegaseous feed.
 9. The method as claimed in claim 8, wherein the anodecomprises nickel.
 10. The method as claimed in claim 9, wherein theeffective temperature is 400° C.
 11. The method as claimed in claim 10,further comprising flowing a second gaseous stream through the secondpassage, the second gaseous stream including oxygen, whilecontemporaneously flowing the first gaseous stream through the firstpassage.
 12. The method as claimed in claim 11, wherein the at least oneoxidizable component is methanol and the concentration of methanol inthe aqueous solution is less than about 2.4% by weight based on thetotal weight of the aqueous solution.
 13. A method of operating a fuelcell including an anode, a cathode, a first passage, and a secondpassage, wherein the anode is disposed in the first passage and thecathode is disposed in the second passage, comprising: (i) progressivelyheating the first fluid passage; (ii) producing a non-explosive gaseousfeed consisting of (a) at least one oxidizable component having agreater tendency to undergo oxidation relative to the anode, and (b) aremainder, wherein the remainder is the predominant component in thegaseous feed and consists essentially of water vapor; and (iii) purgingthe first passage with the gaseous feed when the temperature of theanode is above a temperature effective to cause deteriorative oxidationof the anode in the presence of an oxidizing agent.
 14. The method asclaimed in claim 13, wherein the concentration of the water vapor in thegaseous feed is greater than 50% by volume based on the total volume ofthe gaseous feed.
 15. The method as claimed in claim 14, wherein theconcentration of the at least one oxidizable component is less than theminimum concentration necessary to render the gaseous feed potentiallyexplosive at the effective temperature.
 16. The method as claimed inclaim 14, wherein the concentration of the at least one oxidizablecomponent is less than the lower flammability limit of the at least oneoxidizable component.
 17. The method as claimed in claim 14, wherein thegaseous feed includes the at least one oxidizable component in aconcentration effective to mitigate deteriorative oxidation of theanode.
 18. The method as claimed in claim 17, wherein the gaseous feedincludes the at least one oxidizable component in a concentrationeffective to substantially prevent deteriorative oxidation of the anode.19. The method as claimed in claim 18, wherein the at least oneoxidizable component is selected from the group consisting of hydrogen,alcohols, aldehydes, ketones, ammonia, hydrazine, and hydrocarbons. 20.The method as claimed in claim 19, further comprising evaporating anaqueous mixture consisting essentially of water and the at least oneoxidizable component to produce the gaseous feed.
 21. The method asclaimed in claim 20, wherein the anode comprises nickel.
 22. The methodas claimed in claim 21, wherein the effective temperature is 400° C. 23.The method as claimed in claim 22, further comprising flowing a secondgaseous stream through the second passage, the second gaseous streamincluding oxygen, while contemporaneously purging the first passage withthe gaseous feed.
 24. The method as claimed in claim 23, wherein the atleast one oxidizable component is methanol and the concentration ofmethanol in the aqueous solution is less than about 2.4% by weight basedon the total weight of the aqueous solution.
 25. The method as claimedin claim 24, further comprising, after (iii), terminating the purging ofthe first passage by the gaseous feed and flowing a gaseous fuel throughthe first passage when the temperature within the first fluid passage issufficiently high such that the gaseous fuel is not potentiallyexplosive when disposed in the first passage.
 26. A method of operatinga fuel cell including an anode comprising nickel, a cathode, a firstpassage, and a second passage, wherein the anode is disposed in thefirst passage and the cathode is disposed in the second passage: (i)producing a non-explosive gaseous feed comprising water vapor and atleast one oxidizable component having a greater tendency to undergooxidation relative to the anode by either of: (a) evaporating an aqueousmixture comprising the at least one oxidizable component; or (b)evaporating a source of water to produce the water vapor, and combiningthe water vapor with the at least one oxidizable component; and (ii)purging the first passage with the gaseous feed when the anode realizesa temperature effective to facilitate deteriorative oxidation of theanode in the presence of an oxidizing agent.
 27. The method as claimedin claim 26, wherein the non-explosive gaseous feed is produced byevaporating the aqueous mixture including the at least one oxidizablecomponent.
 28. The method as claimed in claim 27, wherein theconcentration of the water vapor in the gaseous feed is greater than 50%by volume based on the total volume of the gaseous feed.
 29. The methodas claimed in claim 28, wherein the concentration of the at least oneoxidizable component is less than the minimum concentration necessary torender the gaseous feed potentially explosive at the effectivetemperature.
 30. The method as claimed in claim 28, wherein theconcentration of the at least one oxidizable component is less than thelower flammability limit of the at least one oxidizable component. 31.The method as claimed in claim 28, wherein the concentration of the atleast one oxidizable component is effective to mitigate deteriorativeoxidation of the anode.
 32. The method as claimed in claim 31, whereinthe concentration of the at least one oxidizable component is effectiveto substantially prevent deteriorative oxidation of the anode.
 33. Themethod as claimed in claim 32, wherein the at least one oxidizablecomponent is selected from the group consisting of hydrogen, alcohols,aldehydes, ketones, ammonia, hydrazine, and hydrocarbons.
 34. The methodas claimed in claim 33, wherein the anode comprises nickel.
 35. Themethod as claimed in claim 34, wherein the effective temperature is 400°C.
 36. The method as claimed in claim 35, further comprising flowing asecond gaseous stream through the second passage, the second gaseousstream including oxygen, while contemporaneously purging the firstpassage with the gaseous feed.
 37. The method as claimed in claim 36,wherein the at least one oxidizable component comprises methanol. 38.The method as claimed in claim 37, wherein the concentration of methanolin the aqueous solution is less than about 2.4% by weight based on thetotal weight of the aqueous solution.
 39. The method as claimed in claim27, wherein the evaporation is a flash evaporation.
 40. The method asclaimed in claim 38, wherein the evaporation is a flash evaporation. 41.A fuel cell system comprising: a fuel cell including an anode, acathode, a first passage, and a second passage, wherein the anode isdisposed in the first passage and the cathode is disposed in the secondpassage; means for evaporating an aqueous mixture including at least oneoxidizable component to form a gaseous feed; and means for deliveringthe gaseous feed to the first passage to form a first gaseous streamflowing through the first passage and effective in mitigating corrosionof the anode.
 42. A fuel cell system comprising: a fuel cell includingan anode, a cathode, a first passage, and a second passage, wherein theanode is disposed in the first passage and the cathode is disposed inthe second passage: an evaporator, fluidly communicating with the firstpassage, for evaporating an aqueous mixture including at least oneoxidizable component to form a gaseous feed; and a controller,communicating with the fuel cell for receiving an anode corrosionindication, and configured to deliver the gaseous feed to the firstpassage to form a first gaseous stream flowing through the first passagein response to the anode corrosion indication within the fuel cell. 43.The fuel cell system as claimed in claim 42, wherein the controller iscoupled to a temperature sensor for measuring a temperature within thefuel cell, and wherein the controller is configured to effect thedelivery of the gaseous feed at a predetermined temperature.
 44. Thefuel cell system as claimed in claim 43, wherein the controller iscoupled to a motive means configured to effect the delivery of thegaseous feed, and wherein the controller is configured to actuate themotive means to effect the delivery of the gaseous feed at thepredetermined temperature.